The impact is just beginning to be felt: a .5�C warming over the last century. That increase may seem modest, but a 3�C cooling brought on the most recent Ice Age. Not only has the temperature risen on average, but the warming is becoming more intense. The sultriest three years in the century have occurred in the last decade, and the hottest year ever recorded was 1995.

The change is evident not just in thermometer readings. Spring is arriving a week earlier in the Northern hemisphere than it did a decade ago, and strange things are happening with wildlife. A butterfly in the American West known as Edith's checkerspot has moved about 200 km north of where it used to live. Cold-water brook trout have disappeared in some areas of North America as streams warm up. Mountain ice caps in tropical zones are melting fast, as are glaciers in northern climes.

Then there are the unusually frequent bouts of unusually nasty weather: flooding in central Europe, vicious cyclones in South Asia, freak spring snowstorms in the American plains. It's not at all certain that the severity of these storms has anything to do with long-term climate change, but scientists have a possible explanation. If the Earth is indeed warming, some of the heat raises the air temperature a bit, but more of it causes increased evaporation of water. In fact, scientists have found that the amount of moisture in the air has gone up 10% in the last 20 years. The extra moisture could disrupt weather patterns, producing stronger, more frequent storms in some areas and droughts in other places.

The scary thing is that the change has barely started. No one knows how much warming is on the way, but scientists have various models predicting anywhere from a 1�C to 5�C increase by the end of the next century. The effects of a temperature jump in the upper part of that range could dwarf anything that has happened so far. One widely made prediction is a partial melting of the polar ice caps and a consequent rise in sea level that could submerge parts of coastal cities from New York to Bombay. Some island nations like the Maldives could disappear altogether. Scientists concede their atmospheric models are far from perfect, but they have been accurate in predicting telltale signs of human influence on climate, a trick researchers call fingerprinting. For example, the models say that human-caused warming should reduce the gap between daytime high temperatures and nighttime lows and that this narrowing difference should be most apparent in winter. The theory also predicts that while maximum temperatures will rise, minimums will rise at a faster pace. In terms of geographic distribution, warming should be greater in the Northern hemisphere, where most of the greenhouse gases are produced, than in the Southern hemisphere. Finally, there should be vertical variations: the warming will be greatest in the lower portion of the atmosphere and less pronounced in the upper regions.